Russian Prime Minister Dmitry Medvedev has described the explosion between two underground stations in St Petersburg that killed 11 people as a "terrorist attack."

In a post on his Facebook account, the prime minister sent his "most sincere condolences to the families and relatives of the victims of the explosion." He said the injured would be given all the necessary assistance.

An anti-terror investigation has been launched but other possible causes are also being investigated.

A total of 11 people were killed in the blast, Health Minister Veronika Skvortsova said.

Seven died at the scene, and one in the ambulance. At least 45 people were injured and those who were seriously injured have undergone surgery, the minister added.

Six people who are being treated in hospital remain in critical condition. Skvortsova said that one of the victims was a 15-year-old girl who had burns and a head injury.

Andrei Przhezdomsky, the head of the National Anti-Terrorism Committee said that the explosion was caused by "an unidentified explosive device" but that the exact cause has not been determined yet.

Only one explosion between the Sennaya Ploshchad and the Tekhnologichesky Institut underground stations was confirmed at around 2.30pm local time. Another bomb was found and deactivated at the Ploshchad Vosstaniya Station, the Anti-Terrorism Committee said.

The second unexploded bomb was filled with shrapnel and was three times the size of the first bomb, Sky News reported.

Meanwhile President Vladimir Putin, who was in St Petersburg at the time of the blast, has said that all potential causes are being investigated, alongside terrorism.

The leader was later seen arriving at the Tekhnologichesky Institut station to place some commemorative flowers.

Kremlin spokesman Dmitry Peskov added that it was still too early to classify the underground metro blast as a terrorist attack although he admitted that the "signs pointing to a terrorist attack are plain to see."

"From a legal point of view, it is wrong to speak about a terrorist attack just now," he said.